The invention relates generally to high-powered DC power supplies suitable for x-ray tubes and similar high-voltage applications and in particular to a resonant circuit power supply having reduced sensitivity to parameter variations and the like.
Modern x-ray systems use an x-ray tube that accelerate electrons against a target to produce x-rays for therapeutic or imaging purposes. For this purpose, a high-voltage (e.g., 30-150 kilovolts) DC voltage is applied between a tube cathode and anode to provide the necessary electron acceleration. The power supply must be capable of producing current flows at kilowatt power levels.
The spectral content and influence of x-rays depends on the voltage and current applied to the x-ray tube and for this reason it is important that these values be stable.
High-voltage DC/DC (direct current to direct current) converters may be used to convert commonly available line voltage (after rectification and filtering) into the necessary high voltages for these applications. Such DC/DC converters first generate a lower voltage input DC bus by rectification and filtering of a convenient AC line voltage. This bus voltage is then switched by set of semiconductor devices to produce a synthesized AC signal that can be stepped-up in voltage using a transformer. Output from the transformer is then rectified to produce the necessary high voltage direct current. The intermediary steps of synthesizing an AC signal using the semiconductor devices allow feedback control of the output signal by varying a switching sequence of the semiconductor devices according to the output voltage when compared to desired command voltage. For example, the switching sequence of the semiconductor devices can be changed in duty cycle.
Improved efficiency of such DC/DC converters may be obtained through the use of a so-called “resonant converter” design. In such a design, the semiconductor devices produce a synthesized AC signal that is applied to a tank circuit (for example, a series resonant inductor and capacitor) to excite that tank circuit in the oscillation. The resulting AC waveform output from the tank circuit is closely sinusoidal providing improved conversion by the subsequent step-up transformer into a higher voltage AC signal and improved filtration of that higher voltage AC signal.
Despite advances in the resonant DC/DC convertor design, it can be difficult to provide precise voltage control in such designs because of the instability of the parameters of the resonant tank circuit which change over time and nonlinearities in the resonant gain curve of the resonant circuit.